I have heard that they're quite difficult to get into, and they're competitive as well but that also might depend on the program and university. In terms of admission rates, I do know that UBC does, you can find it on their website when you look up their M.A and M.Ed program in Counseling, I'm sure other universities do as well because they do have to report their admission rates.
I think the only way to become a mental health counselor is by obtaining a Master's degree in Counseling or Social Work and meeting the requirements of practicum/internship experience. You can find more about certification on the CCPA website, along with regulatory body within your province, such as the CRPO or BCACC.
You could also look into programs in the U.S., that are CACREP accredited which follow the same guidelines as programs within Canada, for certification.
Hope that helped! And if I'm incorrect about anything, hopefully someone can follow up.